Which solar panels work best in the shade?

When trees or buildings cast partial shadows on a solar panel, it causes power to drop dramatically. Some types of panels are designed to better handle this.

Low light conditions are a normal situation that solar panels are well designed to handle.

Whether it’s cloudy, rainy, or simply early in the morning or late in the afternoon, you should expect your solar panels to continue to generate power. The power output of your solar panels will drop, but with most panels the decrease is fairly linear. That is, if the sunlight drops by half, with most panels the output will also drop by half.

However, a funny situation happens when a solar panel is only partially covered by a shadow. If you block light from half of a panel and leave the other half in bright sunlight, you might expect that the power output will drop by half. But that’s not the case. What actually happens is that the power output will drop dramatically - you will lose much more than half the power.

Exactly how much power a solar panel will lose when partially shaded depends on where the shadow falls on the panel. This is due to a necessity of how the 60, 72, or 96 individual solar cells that make up a panel are wired together. When one solar cell is shaded, it loses power. That loss of power increases electrical resistence within the cell, which in tern reduces the flow of electricity to neighboring cells.

There are two approaches that manufacturers take to build solar panels that work better in partial shade conditions. The first, and most common, is to use an electronic component called a bypass diode. The purpose of a bypass diode is to allow electricity in a solar panel to flow around the shaded cell. A second, less common, and more expensive approach is to use half-cut solar cells. The use of half-cut solar cells increases the available electric pathways in a solar panel, making it more resistant to shade.

A visual explanation of how shading affects solar panels

To help you understand exactly what is meant by partial shading and how it affects the power output of a solar panel, check out this video from the altE store. It’s only five minutes long, but explains the issue really well.

In the video, she shows that the drop in current changes depending on where the shadow falls. This is because the individual cells are wired in series, and where the shadow falls in the series will affect how much current is blocked.

A typical home solar panel has 60 cells wired in series. Here’s a simplied diagram, where the red line represents the wiring that connects the cells:

Diagram of how solar cells are wired together in a panel.

Shaded cells block the flow of electricity, so you can imagine that a shaded cell near the end of the string will be worse than a shadow near the start of it.

Bypass diodes in solar panels

One solution that manufacturers use to improve the performance of solar panels in this situation is to use bypass diodes. A bypass diode is like a detour around a traffic jam: it allows electricity to flow around shaded cells.

All solar panels require bypass diodes for safety reasons, and most have three. This gives a solar panel three possible detours around shaded areas.

Having more detours available will improve solar panel performance in the shade, but adding diodes increases costs. Also, each diode reduces the panel voltage a little bit, so there is a tradeoff. Most panels have three diodes per panel, but some manufacturers may use more. This is not always listed on the panel datasheet, so if this is something you are curious about, you should ask your installer.

For a more detailed explanation about how bypass diodes work, check out this article at Electronics Hub.

Half-cut solar cells

Another approach that some manufacturers use to improve partial shade performance is to use half-cut solar cells. This is exactly what it sounds like: a normal solar cell is cut in two, resulting in a panel with 120 half-cut cells instead of the typical 60.

Additional wiring is used to join the smaller cells. More wiring between the cells provides a greater number of pathways through the panel, which makes it more resistant to partial shading.

In addition, this configuration results in a solar panel that has lower electrical resistance overall, regardless of whether the panel is shaded. This can result in a small increase in overall efficiency - somewhere in the neighborhood of 2%.

Diagram of a solar panel with half-cut solar cells. (Courtesy REC Solar)
Diagram of a solar panel with half-cut solar cells. (Courtesy REC Solar)

As you can see in the diagram above, a solar panel with half-cut cells has two regions, doubling the electrical pathways through the panel. If the top half of the panel is shaded, the bottom will be unaffected.

However, if a shadow falls across both halves, the entire panel will still suffer a power loss, so it’s not a foolproof solution.

Manufacturers making panels with half-cut cells

This is a partial list of manufacturers and their products that feature half-cut cells:

The industry is fast-moving, so ask your installer if their preferred manufacturer has a product line featuring half-cut cells.

Side note: American made half-cut solar panels coming

If you’re interested in buying American-made solar panels, you might want to know that Hanwha (a South Korean company) is building a manufacturing plant in Georgia and will be assembling their Q.Peak DUO G6 panel there.

The website states that assembly will be done in Georgia, but it doesn’t say where the actual cells will be manufactured. That information might become more clear when the product hits the market. In any case, this product is something to keep an eye on if buying American is something that’s important to you.

Disadvantages of half-cut cells

As with anything, there are potential downsides to using half-cut cells.

The first is higher cost due to added manufacturing complexity. First, the cells must be cut (or grooved) using a precision laser. Then, additional wiring and soldering joints must be added to the panel.

The increase in solder points and wiring produces greater opportunities for wiring failures to occur, so half-cut cells may result in lower reliability, although it’s probably too early to tell.

Because of this uncertainty, it might be a good idea to pay particular attention to the product warranty when going with half-cut solar panels.

Of the products listed above, the REC TwinPeak has the best warranty with 20 year product coverage, and a 25 year power warranty. Many other panels have only a 10 year product warranty.

Are half cut cells worth it?

Half-cut cells are just one approach to addressing partial shading conditions and improving solar panel efficiency. Whether it’s worth it really depends on your roof conditions, which is something you should discuss in detail with your installer.

If you have shading conditions only for part of the day, then the cost of a premium panel might just not be worth it. Efficiency is only one consideration when comparing panels, and many times the higher price of a premium panel won’t be recouped by its increased efficiency. Read my article on finding the best solar panels to learn more about how to weigh the different characteristics of solar panels when deciding which one to buy.

Microinverters, power optimizers, and shading

To be clear, this article is about partial shading of a single solar panel, and not shading that affects several solar panels in your array.

Shade that falls across part of your array causes a system-wide problem that is similar to the partial shading of a single solar panel. This is because, just like how individual cells in a panel are wired in series, the panels in an array are also normally wired in series. This means blocking the light on a single panel can affect the entire array.

The solution to this lies with your inverter choice. Microinverters and power optimizers are two inverter technologies that allow your array to better deal with system shading. Both will help maximize your system power output, with the tradeoff of higher cost.

To learn more about inverter choices, read my guide to how solar inverters work.

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